The stability of a large cavern group at great depth is discussed on the basis of large-scale three-dimensional (3-D) geomechanical model tests and numerical simulations. The model tests are described in detail. Improvements in the tests were made in terms of experimental techniques and advanced measurement methods. The model tests utilized active loading on six sides of a rock mass in a true 3-D stress state. During the model construction, precast blocks were fabricated and monitoring holes were defined prior to test initiation. Newly developed combination ball-sliding walls were installed on each of the major loading surfaces to reduce the friction induced by model deformation. A unique grouting and installation technique employing prestressed cables was adopted in the tests. A digital photogrammetric technique, displacement sensing bars using fiber Bragg grating (FBG) technology, and mini multipoint extensometers were developed for measuring deformation. Overloading tests were then conducted for different overburden depths, and 3-D numerical analyses were performed to simulate the testing procedures. Conclusions regarding the stability of the cavern group were developed based on a comparison between the experimental and numerical results.